2. Seasonality of Precipitation
M Hulme

Per cent of precipitation over England and Wales falling in winter (dated by January).

Specific indicator
Monthly precipitation for England and Wales is averaged and quality controlled for 35 representative meteorological stations. The specific indicator chosen is the proportion of England and Wales precipitation falling in winter.
[Source: Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ]

Relevance
The England and Wales Precipitation series extends from 1766 to the present and exists as a series of monthly precipitation (rainfall plus snowfall) totals representative of the mean areal precipitation over England and Wales. The selection of gauges contributing to this series has been carefully scrutinised over time to ensure a homogenous precipitation series. Rather than show here the annual precipitation series, which contains large interannual and interdecadal variability, we choose to show the proportion of annual precipitation falling in winter (December-February).

This index is a measure of how 'Mediterranean' UK climate is - the larger proportion of precipitation falling in winter, the more 'Mediterranean' the climate. The seasonal distribution of precipitation has important implications for how water resources are managed.

Sensitivity to climatic and other factors
This indicator will not reveal overall increases or decreases in the amount of precipitation falling over England and Wales, but it will reveal changes in the seasonal distribution of precipitation. The indicator has been chosen partly because climatic modelling studies suggest that, over the UK, global warming might induce wetter winters.

Change over time
The proportion of precipitation over England and Wales falling in winter has remained quite constant during the present century with a 1961-90 average of about 27 per cent. However, two of the three years with the highest proportions of winter precipitation this century occurred in the 1990s - 1990 and 1995 - although there have also been recent years with quite low proportions - 1992 and 1997.

Recent years have seen proportions slightly above - 1996 - and slightly below - 1997 - the long-term mean. The last four years (1999 – 2002) have been close to the long-term average, with no exceptional occurrences. Although climate modelling studies suggest that we might expect to see higher proportions in future, precipitation, including its seasonal distribution, exhibits large natural variations from year-to-year and from decade-to-decade, and clearly identifying such underlying trends may require many more years of data.